This invention relates to in situ recovery of shale oil, and more particularly, to techniques for forming an in situ oil shale retort with an open space or inlet plenum void between the top boundary of the fragmented permeable mass of oil shale particles in the retort and the unfragmented formation above said retort.
The presence of large deposits of oil shale in the Rocky Mountain region of the United States has given rise to extensive efforts to develop methods for recovering shale oil from kerogen in the oil shale deposits. It should be noted that the term "oil shale" as used in the industry is in fact a misnomer; it is neither shale, nor does it contain oil. It is instead a sedimentary formation comprising marlstone deposit interspersed with layers containing an organic polymer called "kerogen." The kerogen, upon heating, decomposes to produce liquid and gaseous hydrocarbon products. It is the formation containing kerogen that is called "oil shale" herein, and the liquid hydrocarbon product is called "shale oil."
A number of methods have been proposed for processing oil shale which involve either first mining and processing the shale on the surface or processing the shale in situ. The latter approach is preferable from the standpoint of environmental impact, since the treated shale remains in place, reducing the chance of surface contamination and the requirement for disposal of solid wastes.
The recovery of liquid and gaseous products from oil shale deposits have been described in several patents, such as U.S. Pat. Nos. 3,661,423; 4,043,595; 4,043,596,4,043,597; and 4,043,598 which are incorporated herein by this reference. These patents describe in situ recovery of liquid and gaseous hydrocarbon materials from a subterranean formation containing oil shale, wherein such formation is explosively expanded to form a stationary, fragmented permeable body or mass of formation particles containing oil shale within the formation, referred to herein as an in situ oil shale retort. Retorting, gases are passed through the fragmented mass to convert kerogen contined in the oil shale to liquid and gaseous products, thereby producing retorted oil shale. One method of supplying hot retorting gases used for converting kerogen contained in the oil shale includes establishing a combustion zone in the retort and introducing an oxygen-suppling retort inlet mixture into the retort to advance the combustion zone through the fragmented mass. In the combustion zone, oxygen from the retort inlet mixture is depleted by reaction with hot carbonaceous materials to produce heat, combustion gas, and combusted oil shale. By the continued introduction of the retort inlet mixture into the fragmented mass, the combustion zone is advanced through the fragmented mass in the retort.
The combustion gas and the portion of the retort inlet mixture that does not take part in the combustion process pass through the fragmented mass on the advancing side of the combustion zone to heat the oil shale in a retorting zone to a temperature sufficient to produce kerogen decomposition, called "retorting." Such decomposition in the oil shale produces gaseous and liquid products, including gaseous and liquid hydrocarbon products, and a residual solid carbonaceous material.
The liquid products and the gaseous products are cooled by the cooler oil shale fragments in the retort on the advancing side of the retorting zone. The liquid hydrocarbon products, together with water produced in or added to the retort, collect at the bottom of the retort and are withdrawn. An off gas is also withdrawn from the bottom of the retort. Such off gas can include carbon dioxide generated in the combustion zone and from carbonate decomposition, gaseous products produced in the retorting zone, and any gaseous retort inlet mixture that does not take part in the combustion process. The products of retorting are referred to herein as liquid and gaseous products.
U.S. Pat. No. 4,043,598 discloses a method for explosively expanding formation containing oil shale toward horizontal free faces to form a fragmented mass in an in situ oil shale retort. According to a method disclosed in that patent, a plurality of vertically spaced apart voids are initially excavated one above another within the retort site. A plurality of vertically spaced apart zones of unfragmented formation are temporarily left between the voids. Explosive is placed in each of the unfragmented zones and detonated to explosively expand each unfragmented zone into the voids on either side of each unfragmented zone to form a fragmented mass having a void volume substantially corresponding to the void volume of the initial voids. Retorting of the fragmented mass is then carried out to recover shale oil from the oil shale.
It is desirable to establish a combustion zone which is relatively flat and extends laterally across the entire fragmented permeable mass. The relatively flat combustion zone extending laterally across the entire fragmented permeable mass enables retorting of a maximum volume of the fragmented permeable mass of oil shale which, in turn, tends to maximize the yield of liquid and gaseous products from the in situ oil shale retort.
In practice, the combustion zone is created by using one or more burners to ignite a portion or portions of the fragmented permable mass. The burner is ignited and the flame from the burner is directed toward the fragmented permeable mass to heat an upper portion of the fragmented permeable mass to greater than the self ignition temperature of carbonaceous material in the oil shale. The burner is then withdrawn from the retort and fuel and an oxygen-containing gas are introduced to spread the primary combustion zone laterally. Some difficulty has been encountered in causing a combustion zone to spread laterally near the top of an in situ oil shale retort where the fragmented mass completely fills the retort cavity. Thus, considerable time and fuel can be consumed in causing the combustion zone to spread laterally to a sufficient extent to recover shale oil from some of the upper portions of the fragmented mass. Previous techniques have caused the combustion zone to propagate downwardly a considerable distance as it spreads laterally, the downward and radial distance being about the same. This can cause bypass of oil shale in the upper corners of the retort and/or some of the shale oil produced in the upper portions of the fragmented permeable mass to pass through downstream portions of the primary combustion zone thereby being consumed. The consumption of shale oil or the complete bypassing of portions of the oil shale tends to reduce the yield of shale oil from the retorting operation. It is therefore desirable to have a technique for improving the rate and extent of lateral spreading of the combustion zone near the top of the retort.